1. Field of the Invention
The present invention relates to aircraft transparencies, and more particularly, to laminated aircraft transparencies. In the desire to minimize the weight of laminated windows used in aircraft while retaining the penetration resistance of laminated safety glass, various hard plastic materials, such as acrylic plastics, particularly polymethylmethacrylate, and polycarbonates, have been laminated together using various interlayer materials to provide laminated transparencies that have good optical properties and good resistance to impact when a bird collides with the transparency while the aircraft is flying at a high speed.
In fabricating laminated transparencies for aircraft, alternate sheets of relatively rigid plastic, such as acrylic plastic or polycarbonate plastic, are assembled with layers of interlayer material, such as polyurethanes or silicones or plasticized polyvinyl acetals, such as polyvinyl butyral. The assembly is placed within a flexible laminating bag with a pair of pressing plates applied to the outside major surfaces of the assembly to be laminated. The flexible evacuating bag is evacuated and sealed. The bag and its contents are exposed to elevated temperatures and pressures in an autoclave and after such exposure, the flexible bag is cut open and its contents removed. The pressing plates, preferably of chemically tempered glass sheets, are separated from the remainder of the assembly which became laminated.
In the past, the laminating art has had difficulty in avoiding deviations from surface smoothness in the assemblies that were laminated in such a manner. Not only did the major outer surfaces of the assemblies continue to retain surface irregularities, such as small pits or departures from the desired curvature, but also there were small areas of optical distortion which are believed to result from air pockets that remain between the plies of the laminated assembly due to insufficient evacuation of the flexible evacuating bag during the step prior to the final combination press polishing and laminating step.
2. Description of the Prior Art
U.S. Pat. No. 1,725,454 to Heyl discloses a technique for making laminated safety glass comprising adherent sheets of glass and cellulose derivative composition, such as cellulose acetate, in which bubbles of air become trapped between the sheets of the assembly and impair the finished product. This patent requires that the cellulose acetate sheets be moistened to improve their adhesion to glass. To avoid residual air in a closed bag being drawn in between the sheets while the liquid is pressed out by external atmospheric pressure on the bag as the bag is evacuated, liquid is introduced into the bag from the bottom and suction is applied to the top of the bag and the bag and its contents are supported in a vertical orientation during the application of water and the evacuation step.
U.S. Pat. No. 1,909,444 to Worrall applies a frame of non-compressible strips arranged around the perimeter of an assembly of glass and interlayer material to be laminated within a flexible evacuating and laminating bag to prevent excessive pressure upon the edges of the glass.
U.S. Pat. No. 1,959,221 to Sherts applies a pair of wood members provided at their inner edges with wedges. The wedges serve to keep glass plates separated while air is exhausted from a bag containing alternate layers of glass and interlayer material. An operator grasps the wedge members and stretches the rubber bag to retract the wedges so that the sheets come into engagement by gravity and by pressure of the air on the exterior of the bag so that the bag in which the assembly to be laminated is maintained during the application of heat and pressure is in intimate contact and the bag is fully evacuated, thereby eliminating the presence of air bubbles within the bag.
U.S. Pat. No. 2,466,078 to Boicey discloses means for evacuating and sealing a thin flexible bag used to support an assembly of glass and plastic having an extended plastic border.
U.S. Pat. No. 2,783,176 to Boicey discloses a method of laminating acrylic resin to polyvinyl butyral by placing an assembly of layers of acrylic resin and polyvinyl butyral in a bag, exhausting air from the bag, and exposing the bag and its contents to hot water to effect a lamination of the acrylic plastic to the polyvinyl butyral and then hanging the laminate so formed in an oven kept at elevated temperatures to remove wrinkles from the laminate.
U.S. Pat. No. 3,038,825 to Little discloses a flexible bag for laminating glass-plastic assemblies which incorporates rope mounted about three sides of the interior of a bag in which the assembly is laminated. The rope provides an air conduit that prevents complete collapse of the bag top and bottom walls when the bag interior is subjected to vacuum. This feature is alleged to improve the efficiency of evacuation of the laminating bag.
U.S. Pat. No. 3,912,542 to Hirano et al proposes to minimize the presence of air within an assembly to be laminated by interposing an adhesive powdery thermoplastic composition or an air-permeable sintered product thereof between at least two sheet-like materials to form an assembly, maintaining the assembly at reduced pressure within a laminating bag to deaerate the adhesive layer, and heating the assembly while maintaining it at reduced pressure at least at the initial stage of heating to melt the powdery thermoplastic resin composition or the sintered product and bond the sheet-like materials to provide an integral body.